What is the principle of measuring with a digital multimeter?
Digital multimeter, as the name suggests, its working principle is mainly based on digital circuit to detect and analyze signals, and then provide LCD display through analog-to-digital converter.
Although the digital multimeter is a relatively complex electronic measurement tool, if you operate it correctly according to the manual, pay attention to the conventional precautions, and maintain a good operating environment, the service life and accuracy of the instrument will be greatly improved and stable. The digital multimeter uses a liquid crystal display as a reading device, which has the characteristics of high measurement accuracy, safe and reliable use. It has many models and varieties, and the measurement is very simple.
Digital multimeters generally use 7106 type A/D conversion integrated circuits, and some use micro-power 7126 type or their improved 7136 integrated circuits. During the measurement process, they can automatically complete: automatic zero adjustment, signal integration, and reverse integration three. For the cycle of each stage, the count value N is proportional to the measured voltage V, which is expressed by the mathematical formula:
Screen display number N = signal integration time T1 / (clock oscillation period TC × reference voltage V0) × measurement voltage V
In the circuit in the table, the signal integration time is taken as 1000 times of the oscillation period. The clock oscillation period is determined by the value of the clock oscillation RC element connected to the 38, 39, and 40 pins of the 7106 integrated block, and is generally adjusted at a frequency of about 40kHz. The reference voltage is determined by the voltage divider resistors connected to pins 35 and 36, and is generally adjusted at 100.00 mV. Calculated like this: N=1000×V/100.00=10V
Obviously, the measurement result can be read directly by setting the decimal point to ten places. The display value of the most basic range of the 3 1/2 digit digital multimeter is up to 199.99 mV.
The digital square meter measures different DC voltages by resistive voltage division, measures different DC currents by resistive shunts, and measures different AC voltages by resisting voltage division first, and then linearly rectified by operational amplifiers.
The digital multimeter measures resistance using the proportional method. As shown in the figure, after connecting the reference resistance and the resistance under test in series, connect it between the +V of the integrated block and the COM analog ground to obtain the reading. When R0=Rx, the displayed value is 1000. Take the 200Ω block as an example, the reference resistance is 100Ω, the display value is 10Rx, and the decimal point is set to ten, and the result can be directly read. It can be seen that different resistance measurement blocks are realized by changing the resistance value of the reference resistance.
